VolumePolicy: Debounce adjustments from vibrate->silent.

Disallows volume adjustments (lower) from vibrate to silent
until a small period of time after an adjustment from normal
to vibrate.

This provides for enough time for the user to see the state change
to vibrate and feel the associated haptic feedback before yet
another state change to silent.

Bug: 19260237
Change-Id: I5843a1c144d56146a83db194f352832c8d85159d